Comey is set to testify before the Senate Intelligence Committee after Memorial Day, and the main question around his hearing will be whether Comey believes Trump was trying to intervene in the FBI's investigation.

"You have to have intent in order to obstruct justice in the criminal sense," the source told CNN. "Intent is hard to prove."

Trump fired Comey — who was spearheading the Russia probe at the time — two weeks ago. Initially, the White House said that Trump acted entirely based on the advice of deputy attorney general Rod Rosenstein and attorney general Jeff Sessions.

Later, however, Trump said that he had already made the decision to fire Comey before getting Sessions' and Rosenstein's input. He also said that "the Russia thing" had been a factor in his decision, later calling Comey a "showboat," a "grandstander," and a "real nut job."

After Comey was fired, it emerged that he had kept detailed memos of encounters with the president that he deemed improper. One of those memos reportedly details a February meeting in which Trump asked Comey to drop the FBI's investigation into former national security adviser Michael Flynn. Trump reportedly said, according to the memo, that Flynn was "a good guy" and "I hope you can let this go."

The last two weeks have seen a torrent of news that reflects negatively on the White House, including reports that Trump divulged highly classified information to two top Russian officials during a meeting in the Oval Office. In light of these revelations and Comey's firing, Rosenstein appointed former FBI director Robert Mueller as special counsel to investigate ties between Trump's campaign and Russia.

Mueller will have broad powers in his new role and the latitude to investigate anything related to the Trump campaign's ties to Russia and potential collusion with Russia to interfere in the 2016 election.

"Mueller has a reputation for being a straight shooter and won’t be swayed by pressure from the White House," said Jens David Ohlin, an associate dean at Cornell Law School and an expert on criminal law. And although there's no timeline on how long the investigation will take, "it’s not a good development for the Trump White House."